RAFT dispersion polymerisation of lauryl methacrylate in ethanol–water binary mixtures: synthesis of diblock copolymer vesicles with deformable membranes

Abstract

Polymerisation-induced self-assembly (PISA) is widely recognised to be a powerful platform technology for the rational synthesis of diblock copolymer nano-objects. RAFT alcoholic dispersion polymerisation is an important PISA formulation that has been used to prepare block copolymer spheres, worms and vesicles. In this study, we have utilised the RAFT dispersion polymerisation of lauryl methacrylate (LMA) using a poly(N-(2-methacryloyloxy)ethyl pyrrolidone) (PNMEP) stabiliser in order to prepare vesicles with highly deformable membranes. More specifically, a PNMEP₂₈ precursor was chain-extended with LMA in an 80 : 20 w/w ethanol–water mixture to produce a series of PNMEP₂₈-PLMA_x diblock copolymer nano-objects (M_w/M_n ≤ 1.40; LMA conversions ≥98% in all cases, as indicated by ¹H NMR spectroscopy). Differential scanning calorimetry studies confirmed that the membrane-forming PLMA block had a relatively low glass transition temperature. Transmission electron microscopy and small angle X-ray scattering were used to identify copolymer morphologies for these highly asymmetric diblock copolymers. A mixed sphere and vesicle morphology was observed when targeting x = 43, while polydisperse vesicles were obtained for x = 65–151. Slightly smaller vesicles with lower mean aggregation numbers and thicker membranes were obtained when targeting higher PLMA DPs. A minor population of sheet-like lamellae was observed for each target copolymer composition, with lamellar stacking leading to a structure peak in the scattering patterns recorded for PNMEP₂₈-PLMA₁₂₉ and PNMEP₂₈-PLMA₁₅₁. Bearing in mind potential industrial applications, RAFT chain-end removal strategies were briefly explored for such PNMEP₂₈-PLMA_x vesicles. Thus, 96% of dithiobenzoate chain-ends could be removed within 3 h at 50 °C via LED irradiation of a 7.5% aqueous dispersion of PNMEP₂₈-PLMA₈₇ vesicles at a wavelength of 405 nm. This appears to be an attractive method for RAFT chain-end removal from diblock copolymer nano-objects, particularly those comprising highly hydrophobic cores.